Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
In one form of three-dimensional object printing, a radial arm on which one or more printheads are mounted eject building or support material onto a rotating platen or cone to form the object. The printheads eject the material downwardly onto the platen or cone rotating with a constant angular velocity ω. Because the outer perimeter of the rotating structure traverses a greater distance than the inner portions of the structure, the outer perimeter is traveling at a greater velocity than the inner portion. Since the inkjets in the printhead(s) are equally spaced at a distance Δr along the radial arm and each inkjet along the radial arm fires an ink drop having a mass m at time intervals Δt, the density of a solid ring formed by each inkjet is a function of its position along the radial arm. Thus, the density of the ring is approximately m/(rωΔtΔr) since one can assume that Δt and Δr are both relatively small compared to r. The curvature effects from these parameters are second order variations. Consequently, the density of the solid object being formed varies primarily with the position of the inkjet along the radial arm.
One way of keeping the density constant for all inkjets along the radial arm would be to maintain the uniform radial spacing Δr and fire the outer inkjets faster than the inner inkjets. This solution is not implemented in most inkjet printers, however, because inkjet printheads require a constant firing frequency for all inkjets in the printhead. Alternatively, image processing can be designed to compensate for the different radial positions and platen or cone speed differences by halftoning the image data to equalize the density of the material rings formed by the inkjets. This processing, however, wastes a significant amount of the throughput capability of the printheads and introduces various imaging artifacts.
Operating inkjet printers to reduce the variations in object material density caused by variations in the radial position of the inkjets would be beneficial.